Annual report pursuant to Section 13 and 15(d)

PENSIONS AND OTHER POSTRETIREMENT BENEFITS (Estimated Net Periodic Benefit Cost) (Details)

v2.4.1.9
PENSIONS AND OTHER POSTRETIREMENT BENEFITS (Estimated Net Periodic Benefit Cost) (Details) (USD $)
In Millions, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Estimated net periodic benefit cost $ 29.6clf_DefinedBenefitPlanExpectedNetPeriodicBenefitCost  
Pension Benefits [Member]    
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Defined Benefit Plan, Contributions by Plan Participants 0us-gaap_DefinedBenefitPlanContributionsByPlanParticipants
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember 		0us-gaap_DefinedBenefitPlanContributionsByPlanParticipants
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember
Estimated net periodic benefit cost 23.6clf_DefinedBenefitPlanExpectedNetPeriodicBenefitCost
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember 		 
Other Benefits [Member]    
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Defined Benefit Plan, Contributions by Plan Participants 4.8us-gaap_DefinedBenefitPlanContributionsByPlanParticipants
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember 		5.6us-gaap_DefinedBenefitPlanContributionsByPlanParticipants
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
Estimated net periodic benefit cost $ 6.0clf_DefinedBenefitPlanExpectedNetPeriodicBenefitCost
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember